1. Field of the Invention
The present invention relates to a direct positive type silver halide photosensitive material containing chemically fogged silver halide grains and more particularly it relates to a high contrast, direct positive type silver halide photosensitive material having good sensitivity and Dmax, and an improved toe gradient of the characteristic curve.
2. Description of the Prior Art
Direct positive type photosensitive materials utilizing direct positive type silver halide photographic emulsions are advantageous for rapidly and easily obtaining positive images from the originals because they give positive images directly by a single developing process. Direct positive type photosensitive materials are thus widely used for duplication of printing plates, drawings, etc. However, in spite of various endeavors for technical improvements, they still have certain disadvantages, as discussed below.
In producing a direct positive type silver halide photosensitive material, especially a high contrast photosensitive material, when it is attempted to achieve an adequate maximum optical density (Dmax), the gradation in the lower density region (especially where the optical density is less than 1.5) is apt to be low gradient (soft; these are interchangeably used herein in an analogous sense to the use of the term "soft" in The Theory of the Photographic Process, 4th Ed., James, T. H., Ed., 1977, p. 510), thus resulting in inadequate contrast in (poor clearness of; these expressions are interchangeably used herein) the highlight portions. Such photosensitive materials having inadequate contrast in the highlight portions due to the soft gradation in the region of lower optical density are referred to as the photosensitive materials having an undesirable soft toe gradient of the characteristic curve.
In direct positive type light-sensitive materials, fogged nuclei which were formed prior to exposure on the surfaces of silver halide grains are bleached (i.e., oxidized) in the course of the exposure to light, and, upon treatment with a developing solution, the silver halide in the exposed portions is not developed, while the unexposed silver halide is developed. Thus positive images are formed. The fogged nuclei can be formed by the use of a reducing agent and a gold salt, in combination, as disclosed in British Pat. No. 723,019. If it is desired to achieve a sufficiently stable and high Dmax in this type of light-sensitive material, strongly fogged nuclei must be formed over the entire surface of the silver halide grains contained in the emulsions of the light-sensitive materials. When fogged nuclei are formed to meet such a requirement for achieving a sufficiently stable high Dmax, unavoidably some silver halide is formed having excessively fogged nuclei that cannot be bleached (oxidized) during the course of light exposure. As a result, this typical approach in attempting to achieve a high Dmax unavoidably provides direct positive photosensitive materials having low sensitivity and poor contrast in the highlight areas of the image.
The toe gradient in the characteristic curve of the direct positive type silver halide photosensitive emulsion relates to the gradient in the lower density region of the curve (for example, in the region of density of from about 0.1 to 1.0, and especially in the region of density of from 0.1 to 0.5), as is shown in FIG. 1. Therein the curve A shows a characteristic curve having a desirable hard toe gradient and the curve B shows a characteristic curve having an undesirable soft toe gradient.
It is of great concern among those skilled in the art to obtain direct positive type photosensitive materials having an enhanced Dmax and an improved contrast in the highlight portions. The photosensitive material can be evaluated for this by taking the absolute value of the difference between the log E's at D=1.5 and at D=0.1, with the larger value indicating inferior contrast in the highlight portions.
U.S. Pat. No. 3,862,842, issued to Bissonette, relates to amplification of dye images formed utilizing a redox reaction in which metal complex salts are employed as oxidants to act upon conversion of silver images to dye images. In other words, the metal complex salts must necessarily be removed after the silver images are formed as is discussed infra in more detail. Where the metal complex salts are added prior to formation of silver images, a dye image-amplifying effect is not obtained.